Chore: remove unused imports and suppress unused variable warnings

Removes dead use statements across dispatch, formula, import, model, and
UI modules. Prefixes intentionally unused variables with _ in app.rs,
analyzer.rs, and grid.rs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/main.rs

@@ -12,7 +12,7 @@ use std::time::Duration;
 
 use anyhow::{Context, Result};
 use crossterm::{
-    event::{self, Event, KeyCode, KeyModifiers},
+    event::{self, Event},
     execute,
     terminal::{disable_raw_mode, enable_raw_mode, EnterAlternateScreen, LeaveAlternateScreen},
 };